The invention generally relates to a process for improving the mechanical properties of aramid fibers. In particular, the invention relates to a process for improving the flexural rigidity of aramid fibers by depositing a diamond-like-carbon coating on said fibers. This invention is the result of a contract with the United States Department of Energy (Contract No. W-7405-ENG-36).
Since at least the early 1960""s liquid crystalline polymers have been used to produce high strength fibers. Well known examples of these types of fibers include aramid fibers made from highly-oriented rod-like polymers of poly(paraphenylene terephthalamide), well known as KEVLAR(copyright) aramid fibers commercially available from E. I. du Pont de Nemours and Company, Wilmington, Del. or TWARON(copyright) fibers, commercially available from AKZO Nobel NV, Netherlands. These aramid fibers provide exceptional tenacity and a high tensile modulus. Breaking strengths of 2.3-3.4 GPa, with a modulus of 55-143 GPa, are typical for these fibers. This, combined with their low specific gravity and thermal stability, has resulted in improved performance in many structural applications such as aircraft, boats, sporting goods, missiles and armor. However, a major draw back with these types of fibers has been their relatively poor flexural rigidity and compressive properties. Fibers yield at low values of stress on the order of 400 MPa with the formation of kink bands.
In order to alleviate this difficulty, much effort has gone into attempts to cross-link the polymer in the filaments to improve mechanical properties, but to date there has been little success. Another approach has been to coat the fiber with a sufficiently high modulus material to, in effect, xe2x80x9cgirdlexe2x80x9d the filaments preventing buckling. Early work by McGarry et al. (see F. J. McGarry et al., SAMPE Quarterly, p. 35, July 1993) demonstrated the effectiveness of this approach with vapor deposited alumina coatings. Recently, enhanced properties have been reported for the microwave plasma assisted organometallic deposition of TiN coatings on KEVLAR(copyright) aramid fibers.
Aramid fibers have also been coated with a thin, smooth, adherent isotropic carbon coating for use as medical sutures where biocompatability is important (see U.S. Pat. No. 4,204,542 (Bokros et al.)). The carbon coating is deposited on the fiber substrate using vacuum-vapor deposition (VVD). Moreover, diamond-like-carbon coatings have also been deposited on aramid fibers by initially pre-coating the fibers with a thin nickel layer to confer conductivity (see WO 95/22169 (Blanchet-Fincher et al.)). However, although the direct coating of aramid fibers is not straightforward, it is desirable to coat such fibers directly without the need for any intermediate metal layer in order to improve the mechanical properties of the aramid fibers.
In order to overcome the deficiencies and disadvantages of the prior art, what is needed is a process for improving the mechanical properties, in particular the flexural rigidity, of aramid fibers. Other objects and advantages of the present invention will become apparent to those skilled in the art upon reference to the attached figures and the detailed description of the invention which hereinafter follows.
The present invention provides a process for improving the mechanical properties, preferably the flexural rigidity, of aramid fibers. The process comprises directly coating an aramid fiber with a high modulus, high strength material such as diamond-like-carbon.
The invention also provides for diamond-like-carbon coated aramid fibers having improved mechanical properties, (e.g., flexural rigidity), compared to uncoated aramid fibers. When diamond-like-carbon (DLC) coatings are applied directly to aramid fibers, there is an increase in flexural rigidity and suppression of low yielding behavior when the fibers are tested and their mechanical properties evaluated.